Frequency modulation



" amplifier I.

Patented July -7, 1942 FREQUENCY MODULATION Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 10, 1940, Serial No. 360,569 6 Claims. j (CLAUS-171.5)

present invention relates to the art of frequency modulation. One object of my invention is to provide an improved frequency modulation transmitter in which the mean transmitted or carrier frequency is maintained highly constant.

Briefly, in carrying out my invention waves from a frequency modulated oscillator are multiplied in frequency to a value greater than that desired for transmission. Another portion of the waves from. the frequency modulated oscillator is beat down with waves from -a local auxiliary oscillator. The beat waves are substantially stripped of side bands and theresulting 'isolated beat frequency carrier is then heterodyned up in frequency with another portion of waves from the auxiliary local'generator. This higher frequency beat is then `multlpliedfin fre- -The beat' frequency appearing in the output of converter I2 is fed to a low pass filter I l. vThe width of the filter Ill is `such as 'to pass substantially only the mean frequency appearingin the output of converter I2 and to strip all side ban`d frequencies from the beat Waves appearing in the output of converter I2.

quency and after multiplicationV combined with the modulated waves which were previously multiplied toavalue too high for transmission. The

combined waves are then utilized for radiation. This system offers many advantages which will be apparent as the further description thereof proceeds. The latter will be givenwith the ald of the accompanying drawing in which Flg'ure 1 is a schematic diagram of my improved trans-` mitter and Figure 2 is a modification of the arrangement of Figure 1.

Referring to Figure 1 of the drawing, voice or music impressed upon the microphone 2 is amplified by the voice or modulation frequency V'.l'hese amplified signal waves are used to vibrate a crystal electrode of'a crystal controlledoscillation generator 8, thereby varyingA the frequency thereof. If desired, of course, the amplified-waves from amplifier I ,may be used to vary a reactance tube in shunt to the crystal of the oscillation generator 8 or a reactance tube in shunt to thetuned plate circuit of the l crystal controlled cscillationgenerator 8. 'I'he frequency modulated waves from the frequency modulated crystal controlled oscillator 8 having a mean frequency F are fed to a frequency multiplier 8 and multiplied to a high frequency.- AMultiplier 8 `also serves to `increase the amount The' output of low pass filter Il is fed to a second converter I8 which is also fed through lead with oscillationsfrom the local generator I6. By means of tuned circuits in the converter I8, as well as tuned circuits in the amplifier and limiter 2I, the frequency F is selected, amplified and limited and then fed through lead 22. to a frequency multiplier 24.

The frequency multiplying factor of multiplier 24 is less than that of multiplier 8 so that when the wave outputs of multipliers 8 and 24 are beat together in converter 28, the output therefrom of frequency modulation to a desired degree.V

Another portion of the crystal controlled oscillator tube output is fed through lead ill to a detector or converter I2. The latter is supplied through lead Ill with oscillations from a crystal controlled or other controlled oscillationg'en-v erator I8, the frequency of which is approximately equal to the mean frequency 1" of the crystal controlled oscillation generator l.

appearing'in line 28 is of asuitable radiation frequency.

The output of converter 26 appearing in line 28 is amplifiedby amplifier 30 and then fed to a suitable radiating antenna 22. If desired, of cou/rte,v the output of the converter 26 may be frequency multiplied, as well as amplified, before it is radiated over the antenna 82. .Y A

It will be understood that an amplifier and/or .limiter may be interposed between converters I2 and I8 instead of at 2|, or in addition to the amplifier and limiter at 2l;

It is to be cl rly understood-that apparatus t may represen a'phase modulated transmitter rather than a 'frequency modulated transmitter. The rest of the system, however, 'may remain the same. The term timing modulation is used in the appended claims to include either frequency or phase modulation.

tion type, theA. F. amplifier 'l may havea gain versus frequency characteristic that rises with frequency, as it has been found that in any' method of audio frequency transmission the noise is vreduced by pre-emphasizing the higher frequencies before transmission and correspond- F. M. receiverinciuding means for rie-emphasiz- `ing higher audio frequencies, then amplifier Il is preferably arranged to provide pre-emphasis of substantially only the lower audio frequencies rather than to provide gain which is inversely If the transmitter is of the frequency modula-` nected directly to multiplier 24 wherein th'e wave energy is multiplied to a nal frequency less than that produced by multiplier 8. The output of multiplier 24 is then supplied to a carrier filter 25 adapted to pass only the carrier frequency. The output of multiplier 24 and its filter 25 in this case, together with the output of multiplier t, may, as before, be fed directly to .converter 26, the output of which may be subsequently utilized as desired.

It is realized that it is impractical to obtain a strictly unmodulated carrier by simple filtration as suggested above, but useful results may be obtained so long as the filtration is suilcient to bring out the carrier component strongly in comparison with the side bands. s

Havingthus described my invention, what I 1. The method 'of signaling which includes producing a wave modulated as to its timing, in

one path multiplying the modulated waves fre..l

quency to a value higher than that to be transmitted, in another'path beating the modulated wave to some lower frequency, ltering the waves so beat down, heterodyning the filtered wave tov 'aaeaoaicombination with the frequency modulated wave which has been multiplied in frequency.

3. A transmitter comprising an oscillation generator, means for modulating the timing of the.

-' converter to produce a lower beat frequency, a

ilter for filtering the beat frequency output of said converter, and means for combining and utilizing waves derived from said filter and waves derived from said multiplier'.

4. Apparatus as claimed in the preceding claim, characterized by the provision of means for heterodyning the output of said lter with another.

portion' of waves from said auxiliary oscillator to a higher frequency prior to utilization.

5. Apparatus asclaimed in claim 3, characterized by the provision of means for heterodyning the output of said filter with anotherportion of waves from said auxiliary oscillator to a higher frequency prior to utilization, andfurther characterized by the fact that a frequency multiplier is provided for frequency multiplying the heteroy -dyned waves prior to utilization.

6. In a system for increasing the modulation of a carrier wave the timing of which is modulated in accordance with signals, a source of carrier wave energy so modulated, a mixing stage, a first path coupling Said source yto said mixing stage, a wave frequency multiplier in said pathv 

